Increased sensitivity photocathode structure

ABSTRACT

A photocathode substrate includes a plurality of prisms to refract radiation before it impinges upon a photocathode material. The refraction causes most of the radiation to meet the photocathode material at an angle other than 90* so that internal reflection of the energy within the photocathode material occurs. This greatly enhances the probability of electrons being released from the photocathode material.

United States Patent Goodrich et al. I Q

[ 1 Oct. 24, 1972 i [54] INCREASED SENSITIVITY PHOTOCATHODE STRUCTURE [72] Inventors: George W. Goodrich, Bloomfield Hills; William B. Colson, Troy; Calvin C. Matle, Dearborn, all of Mich.

[73] Assignee: The Bendix Corporation [22] Filed: April 8, 1971 21 Appl. No.: 132,372

52] us. Cl. ..313/103, 250/213 v'r, 250/216 51 Int. Cl ..H0lj 43/00 58 FieldofSearch ..250/2l6,213 12,213 VT;

[56] References Cited UNITED STATES PATENTS Krusewl ..250/213 R 4/1962 Salinger ..250/213 VT Primary Examiner-James W. Lawrence Assistant Examiner-D. C. Nelms Attomey-Lester L. Hallacher and Flame, ,l-lartz, Smith & Thompson [57] ABSCT A photocathode substrate includes a plurality of prisms to refract radiation before it impinges upon a photocathode material. The refraction causes most of the radiation to meet the photocathode material at an angle other than 90 so that internal reflection of the energy within the photocathode material occurs. This greatly enhances the probability of electrons being released from the photocathode material.

11 Claims, 1 Drawing Figure P'ATTENTEDncm m2 INVENTORS GEORGE W. GOODRICH WILLIAM B. COLSON CALVIN CMATLE ATTORNEY INCREASED SENSITIVITY PHOTOCATHODE STRUCTURE BACKGROUND OF THE INVENTION The use of photocathode materials to detect radiation such as electromagnetic energy is well known in the art. In such devices, radiation incident upon the photocathode material causes the release of electrons which are then detected and multiplied to yield an output indicative of the incident radiation Two types of photocathode structures are commonly in use. The first employs an opaque photocathode material so that the impinging radiation impacts the photocathode material on the surface from which the electrons are emitted. The other type employs a transparent photocathode material so that the radiation impinges the material on the surface opposite from that from which the electrons emanate.

In either of these types of photocathodes, and particularly in the transparent type, it is preferable to have the radiation impact the photocathode material at an angle other than 90. This is so because a 90 incidence of the radiation has a high probability of permitting the radiation to pass directly through the photocathode material without the release of electrons. This detrimental effect is increased in transparent photocathode materials because the layer of photocathode material must be quite thin in order to prevent absorption of released electrons by the photocathode material.

The requirement for having the radiation impact the photocathode material at an angle is at odds with the requirement of having parallel rays of energy impact the photocathode material. The requirement for parallel rays of energy arises from the fact that usually the radiation to be detected contains image information. The object of the photocathode material is to emit electrons indicative of this image information. Accordingly, the radiation isusually focused onto the photocathode substrate in order to maximize the preservation of the image information. The focusing causes a substantial portion of the energy to impact the substrate at an angle of incidence of very nearly 90.

SUMMARY OF THE INVENTION The invention overcomes the disadvantages of the prior art systems in that it provides a photocathode substrate which refracts the incident radiation so that most of the energy strikes the photocathode material at an angle beyond the critical angle required for internal reflection irrespective of the angle of incidence of the energy on the substrate surface. Because of the angular incidence of the radiation upon the photocathode material, the energy undergoes total and multiple internal reflection within the photocathode material. This greatly enhances the probability of an electron release during one of the reflections.

The angular incidence of the radiation with respect to the photocathode material is achieved by the provision of a plurality of substantially parallel arranged longitudinal prisms. The prisms are positioned such that their apexangles face the surface upon which the radiation impinges. Radiation which would ordinarily impact with the photocathode material at a less optimum angle is therefore slightly refracted so that it impacts the photocathode material at a more optimum angle, thereby increasing the probability of the release of an electron by the energy.

Resolution of the image information contained within the incident radiation is preserved because the total refraction of the radiation is very small compared with the typical resolution capabilities of the electron detector associated with the photocathode structure. Also, the resolution along the longitudinal axes of the prisms is essentially the same as that along the crosssectional dimensions of the prisms because the internal reflection of the radiation within the photocathode material will occur for a length which is approximately equal to the cross-sectional dimension of the prisms. This is so because after several reflections the radiation has either released an electron or is attenuated to an intensity level which is insufficient to release an electron.

The inventive photocathode structure is advantageous because multiple, total internal reflection of the energy takes place in the photocathode material. The invention is also advantageous because the total internal reflection occurs even though the photocathode material is a flat, planar surface instead of an irregular configuration as is usually used with photocathode structures attempting to improve the internal reflection of energy within the photocathode material.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE shows a cross-sectional view of a preferred embodiment of the invention.

DETAILED DESCRIPTION In the FIGURE the inventive substrate structure includes a glass Input Substrate 10 having an Input Surface 11 upon which the radiation to be detected impinges. Input Surface 1 1 is planar and Input Substrate 10 is transparent to the incident radiation. Therefore radiation impacting Input Surface 11 at nearly right angles will pass directly through glass Substrate 10. However, radiation impacting at an angle will be refracted upon exit at Output Surface 12 because of the difference in the index of refraction between Substrate 10 and its surrounding mediums. A plurality of Prisms 13 are arranged such that their Apexes 14 are adjacent to Output Surface 12 of the Input Substrate 10. Prisms 13 are therefore arranged such that their Bases 15 form a flat, planar surface. The structural strength of the substrate can be increased by adding a Support Layer 16 of glass or other material which is transparent to the radiation to be detected.

The arrangement of Prisms 13 with respect to Input Substrate 10 results in another plurality of Prisms 17 having their apexes facing downwardly; that is, diametrically opposed to the Apexes 14 of Prisms 13. Prisms 17 can be hollow so that they contain air, and accordingly have an index or refraction equal to unity. Alternatively, if desired, an energy transparent material can be used. Preferably the material used for Prisms 17 will have an index of refraction which is equal or very close to unity. However, the primary requirement is that the index of refraction of Prisms 17 be less than that of Prisms 13.

A layer of Photocathode Material 18 is applied to the bases of Prisms 13. Alternatively, if Support Substrate 16 is used, the photocathode material is applied to the output surface of Supporting Substrate 16.

The radiation to be detected impinges upon Input Surface 11 and passes through the input material until it exits at Output Surface 12 where it enters Prisms 17.

The energy then enters the glass Prisms 13 where refraction occurs so that in most instances the energy impacts Photocathode Material 18 at an angle other than 90. Because of the angular incidence of the radiation with respect to Photocathode Material 18, the radiation undergoes multiple internal reflections within the photocathode material as indicated by Reference Numeral 19. Because of the multiple and total internal reflection of the radiation within the photocathode material, the probability that electrons will be released is very high.

Electrons, generally indicated by Reference Numeral 20, leave Photocathode Surface 18 and are attracted to an Electron Detector 21 which is at a much higher positive potential than Photocathode Material 18. The electron detector can, for example, be a microchannel plate or some other type of electron detector chosen from the many existing types known to those skilled in the art. Because the detection of electrons is essential for the operation of the system, the Region 22 lying between Photocathode Material 18 and Electron Detector 21 is ordinarily evacuated so that the electrons do not meet interference from the ambient at mosphere.

An increase in sensitivity of the inventive photocathode structure in comparison to the prior art substrate structures is primarily occasioned by the total internal reflection of the incident radiation. This internal reflection is caused by the angular incidence of the energy with the photocathode material. Rays impinging upon the prism faces at most angles will undergo total internal reflection at the cathode surface providing the condition:

(n l)" cosasina 1 is satisfied where:

n the index of refraction of Prisms 13 a the half apex angle of Prisms 13 This inequality is satisfied for any glass with an index greater than Vfby the appropriate choice of a. When this condition is met even energy rays striking a prism surface at a grazing angle will undergo some internal refraction and therefore, in all probability, impact the photocathode material at an angle and cause the release of electrons from Photocathode Layer 18.

Total internal reflection within Photocathode Material 18 is also enhanced when the indexes of refraction of the Input Substrate 10 and Support Substrate 16 are low and the index of refraction of the Photocathode Material 18 is high. Furthermore, as stated hereinabove, a preferable index of refraction of the inverted Prisms 17 is unity, although it can be the same as that of Substrates l and 16.

Although the incident radiation is refracted by the inventive photostructure substrate, image resolution is enhanced as compared with that of many existing photocathode structures. This is so because the photocathode material layer is planar, and this prevents electrons emanating from the photocathode from being influenced by contours in the photocathode substrate during the early part of their trajectories. It should also be noted that resolution is preserved because the resolution of the inventive substrate is better than that of the Electron Detector 21 ordinarily associated with such substrates. This can be understood by viewing the FIGURE where the Electrons 20 emanate from Photocathode Material 18 with an initial velocity which ordinarily is not perpendicular to the substrate surface. Accordingly, the electrons have a tendency to travel in arcuate paths through Evacuated Space 22 as generally indicated by Lines 23. Because electrons are attracted to Photocathode Detector 21 along Curved Paths 23, they do not impact Electron Detector 21 at a point which is directly opposite'the point on Photocathode Material 18 from which they emanated. In prior art photocathode substrates, in which the photocathode material is not planar, the contour of the photocathode material affects the trajectory of the electrons, and therefore their arcuate travel through Space 22 exceeds the electrons leaving theinventive planar substrate.

Another reason resolution is not detrimentally affected by the refraction within Prisms 13 stems from the fact that the resolution capabilities of Electron Detector 21 are physically limited. Assuming, for example, that Electron Detector 21 is a microchannel plate, all electrons detected by the MCP enter one of the channels within the plate and cause the emission of secondary electrons. Any electron entering a particular channel will result in an indication at the output of the channel irrespective of the point of origin of the electron from Photocathode Substrate 16. As a consequence, there is a loss of resolution between the Photocathode Material 18 and the Electron Detector 21. In the inventive system, this loss of resolution exceeds that produced by the refraction of the energy through the substrate, and therefore no resolution is lost by use of the inventive system. Furthermore, in the prior art devices employing contoured photocathode materials, the loss of resolution across the evacuated Region 22 exceeds that of the instant invention, and therefore the loss of resolution in the prior art devices exceeds that of the inventive system. A major advantage of the inventive structure is therefore seen to be a planar configuration for the Photocathode Material 18 which also allows total internal reflection within photocathode material.

' It will be appreciated that the preferred embodiment shown in the FIGURE is greatly enlarged and that the height of prisms 13 from their base to the Apex angle will ordinarily be in the order of a few 0.001 of an inch. The thickness of Input Surface 11, Support Surface 16 and Photocathode Material 18 will also be much smaller than those presented in the illustration. For ex ample, the photocathode material 18 will be in the order of a few microns in thickness while Support Surface 16 will be a few 0.001 of an inch.

What is claimed is:

1. A- photocathode structure for detecting radiation comprising:

an input surface transparent to said radiation;

a planar photocathode surface in the proximity of said input surface;

a plurality of refractive elements interposed between said input surface and said photocathode surface, said refractive elements refracting radiation emerging from said input surface away from a perpendicular disposition with respect to said photocathode surface to increase internal reflection of said radiation and enhance the probability that electrons will photocathode surface.

2. The photocathode structure of claim 1 wherein said refractive elements are longitudinal elements arranged in a parallel relationship so that substantially all of said radiation passes through said elements.

3. The photocathode structure of claim 2 wherein said refractive elements have an index of refraction greater than unity.

4. The photocathode structure of claim 2 wherein said refractive elements are longitudinal prisms, the bases of said prisms forming a planar surface for supporting said photocathode surface.

5. The photocathode structure of claim 4 wherein said input surface is supported by the apexes of said prisms; and including a second plurality of prisms between said longitudinal prisms, the index of refraction of said second plurality of prisms being approximately unity.

6. The photocathode structure of claim 5 wherein said longitudinal prisms are constructed in accordance with the constraint:

be emitted by said where:

n index of refraction of the prisms a half apex angle of the prisms. 7. The photocathode structure of claim 1 wherein said refractive elements are longitudinal prisms having a triangular cross-section, the apexes of said prisms supporting said input surface; a second plurality of prisms between said longitudinal prisms and said input surface, the index of refraction of said second prisms being approximately unity.

8. The photocathode structure of claim 7 wherein said input surface has an index of refraction n said photocathode surface has an index of refraction n and n is greater than n 9. The photocathode structure of claim 7 wherein said longitudinal prisms are fabricated in accordance with the constraint:

(n 1) cos a-sin 01 l where:

n index of refraction of the longitudinal prisms a =half apex angle of longitudinal prisms.

10. The photocathode structure of claim 9 further including a support substrate interposed between said photocathode layer and the bases of said prisms.

11. The photocathode structure of claim 10 wherein said input surface and said support substrate are formed from glass, said second plurality of prisms are composed of air, andsaid longitudinal prisms have an index of refraction greater than 

1. A photocathode structure for detecting radiation comprising: an input surface transparent to said radiation; a planar photocathode surface in the proximity of said input surface; a plurality of refractive elements interposed between said input surface and said photocathode surface, said refractive elements refracting radiation emerging from said input surface away from a perpendicular disposition with respect to said photocathode surface to increase internal reflection of said radiation and enhance the probability that electrons will be emitted by said photocathode surface.
 2. The photocathode structure of claim 1 wherein said refractive elements are longitudinal elements arranged in a parallel relationship so that substantially all of said radiation passes through said elements.
 3. The photocathode structure of claim 2 wherein said refractive elements have an index of refraction greater than unity.
 4. The photocathode structure of claim 2 wherein said refractive elements are longitudinal prisms, the bases of said prisms forming a planar surface for supporting said photocathode surface.
 5. The photocathode structure of claim 4 wherein said input surface is supported by the apexes of said prisms; and including a second plurality of prisms between said longitudinal prisms, the index of refraction of said second plurality of prisms being approximately unity.
 6. The photocathode structure of claim 5 wherein said longitudinal prisms are constructed in accordance with the constraint: (n2 - 1)1/2 cos Alpha - sin Alpha > 1 where: n index of refraction of the prisms Alpha half apex angle of the prisms.
 7. The photocathode structure of claim 1 wherein said refractive elements are longitudinal prisms having a triangular cross-section, the apexes of said prisms supporting said input surface; a second plurality of prisms between said longitudinal prisms and said input surface, the index of refraction of said second prisms being approximately unity.
 8. The photocathode structure of claIm 7 wherein said input surface has an index of refraction n1, said photocathode surface has an index of refraction n2 and n2 is greater than n1.
 9. The photocathode structure of claim 7 wherein said longitudinal prisms are fabricated in accordance with the constraint: (n2 - 1)1/2 cos Alpha - sin Alpha > 1 where: n index of refraction of the longitudinal prisms Alpha half apex angle of longitudinal prisms.
 10. The photocathode structure of claim 9 further including a support substrate interposed between said photocathode layer and the bases of said prisms.
 11. The photocathode structure of claim 10 wherein said input surface and said support substrate are formed from glass, said second plurality of prisms are composed of air, and said longitudinal prisms have an index of refraction greater than Square Root
 2. 